Re: [PATCH] sched: pull tasks when CPU is about to run SCHED_IDLE tasks

[Jiang Biao]

Hi,

On Wed, 3 Feb 2021 at 19:17, chin <ultrachin@xxxxxxx> wrote:
>
>
>
>
> At 2021-02-02 23:54:15, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> >On Tue, 2 Feb 2021 at 08:56, chin <ultrachin@xxxxxxx> wrote:
> >>
> >>
> >>
> >>
> >> At 2021-01-13 16:30:14, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> >> >On Wed, 13 Jan 2021 at 04:14, chin <ultrachin@xxxxxxx> wrote:
> >> >>
> >> >>
> >> >>
> >> >>
> >> >> At 2021-01-12 16:18:51, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> >> >> >On Tue, 12 Jan 2021 at 07:59, chin <ultrachin@xxxxxxx> wrote:
> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >> At 2021-01-11 19:04:19, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> >> >> >> >On Mon, 11 Jan 2021 at 09:27, chin <ultrachin@xxxxxxx> wrote:
> >> >> >> >>
> >> >> >> >>
> >> >> >> >> At 2020-12-23 19:30:26, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> >> >> >> >> >On Wed, 23 Dec 2020 at 09:32, <ultrachin@xxxxxxx> wrote:
> >> >> >> >> >>
> >> >> >> >> >> From: Chen Xiaoguang <xiaoggchen@xxxxxxxxxxx>
> >> >> >> >> >>
> >> >> >> >> >> Before a CPU switches from running SCHED_NORMAL task to
> >> >> >> >> >> SCHED_IDLE task, trying to pull SCHED_NORMAL tasks from other
> >> >> >> >> >
> >> >> >> >> >Could you explain more in detail why you only care about this use case
> >> >> >> >>
> >> >> >> >> >in particular and not the general case?
> >> >> >> >>
> >> >> >> >>
> >> >> >> >> We want to run online tasks using SCHED_NORMAL policy and offline tasks
> >> >> >> >> using SCHED_IDLE policy. The online tasks and the offline tasks run in
> >> >> >> >> the same computer in order to use the computer efficiently.
> >> >> >> >> The online tasks are in sleep in most times but should responce soon once
> >> >> >> >> wake up. The offline tasks are in low priority and will run only when no online
> >> >> >> >> tasks.
> >> >> >> >>
> >> >> >> >> The online tasks are more important than the offline tasks and are latency
> >> >> >> >> sensitive we should make sure the online tasks preempt the offline tasks
> >> >> >> >> as soon as possilbe while there are online tasks waiting to run.
> >> >> >> >> So in our situation we hope the SCHED_NORMAL to run if has any.
> >> >> >> >>
> >> >> >> >> Let's assume we have 2 CPUs,
> >> >> >> >> In CPU1 we got 2 SCHED_NORMAL tasks.
> >> >> >> >> in CPU2 we got 1 SCHED_NORMAL task and 2 SCHED_IDLE tasks.
> >> >> >> >>
> >> >> >> >>              CPU1                      CPU2
> >> >> >> >>         curr       rq1            curr          rq2
> >> >> >> >>       +------+ | +------+       +------+ | +----+ +----+
> >> >> >> >> t0    |NORMAL| | |NORMAL|       |NORMAL| | |IDLE| |IDLE|
> >> >> >> >>       +------+ | +------+       +------+ | +----+ +----+
> >> >> >> >>
> >> >> >> >>                                  NORMAL exits or blocked
> >> >> >> >>       +------+ | +------+                | +----+ +----+
> >> >> >> >> t1    |NORMAL| | |NORMAL|                | |IDLE| |IDLE|
> >> >> >> >>       +------+ | +------+                | +----+ +----+
> >> >> >> >>
> >> >> >> >>                                  pick_next_task_fair
> >> >> >> >>       +------+ | +------+         +----+ | +----+
> >> >> >> >> t2    |NORMAL| | |NORMAL|         |IDLE| | |IDLE|
> >> >> >> >>       +------+ | +------+         +----+ | +----+
> >> >> >> >>
> >> >> >> >>                                  SCHED_IDLE running
> >> >> >> >> t3    +------+ | +------+        +----+  | +----+
> >> >> >> >>       |NORMAL| | |NORMAL|        |IDLE|  | |IDLE|
> >> >> >> >>       +------+ | +------+        +----+  | +----+
> >> >> >> >>
> >> >> >> >>                                  run_rebalance_domains
> >> >> >> >>       +------+ |                +------+ | +----+ +----+
> >> >> >> >> t4    |NORMAL| |                |NORMAL| | |IDLE| |IDLE|
> >> >> >> >>       +------+ |                +------+ | +----+ +----+
> >> >> >> >>
> >> >> >> >> As we can see
> >> >> >> >> t1: NORMAL task in CPU2 exits or blocked
> >> >> >> >> t2: CPU2 pick_next_task_fair would pick a SCHED_IDLE to run while
> >> >> >> >> another SCHED_NORMAL in rq1 is waiting.
> >> >> >> >> t3: SCHED_IDLE run in CPU2 while a SCHED_NORMAL wait in CPU1.
> >> >> >> >> t4: after a short time, periodic load_balance triggerd and pull
> >> >> >> >> SCHED_NORMAL in rq1 to rq2, and SCHED_NORMAL likely preempts SCHED_IDLE.
> >> >> >> >>
> >> >> >> >> In this scenario, SCHED_IDLE is running while SCHED_NORMAL is waiting to run.
> >> >> >> >> The latency of this SCHED_NORMAL will be high which is not acceptble.
> >> >> >> >>
> >> >> >> >> Do a load_balance before running the SCHED_IDLE may fix this problem.
> >> >> >> >>
> >> >> >> >> This patch works as below:
> >> >> >> >>
> >> >> >> >>              CPU1                      CPU2
> >> >> >> >>         curr       rq1            curr          rq2
> >> >> >> >>       +------+ | +------+       +------+ | +----+ +----+
> >> >> >> >> t0    |NORMAL| | |NORMAL|       |NORMAL| | |IDLE| |IDLE|
> >> >> >> >>       +------+ | +------+       +------+ | +----+ +----+
> >> >> >> >>
> >> >> >> >>                                  NORMAL exits or blocked
> >> >> >> >>       +------+ | +------+                | +----+ +----+
> >> >> >> >> t1    |NORMAL| | |NORMAL|                | |IDLE| |IDLE|
> >> >> >> >>       +------+ | +------+                | +----+ +----+
> >> >> >> >>
> >> >> >> >> t2                            pick_next_task_fair (all se are SCHED_IDLE)
> >> >> >> >>
> >> >> >> >>                                  newidle_balance
> >> >> >> >>       +------+ |                 +------+ | +----+ +----+
> >> >> >> >> t3    |NORMAL| |                 |NORMAL| | |IDLE| |IDLE|
> >> >> >> >>       +------+ |                 +------+ | +----+ +----+
> >> >> >> >>
> >> >> >> >>
> >> >> >> >> t1: NORMAL task in CPU2 exits or blocked
> >> >> >> >> t2: pick_next_task_fair check all se in rbtree are SCHED_IDLE and calls
> >> >> >> >> newidle_balance who tries to pull a SCHED_NORMAL(if has).
> >> >> >> >> t3: pick_next_task_fair would pick a SCHED_NORMAL to run instead of
> >> >> >> >> SCHED_IDLE(likely).
> >> >> >> >>
> >> >> >> >> >
> >> >> >> >> >> CPU by doing load_balance first.
> >> >> >> >> >>
> >> >> >> >> >> Signed-off-by: Chen Xiaoguang <xiaoggchen@xxxxxxxxxxx>
> >> >> >> >> >> Signed-off-by: Chen He <heddchen@xxxxxxxxxxx>
> >> >> >> >> >> ---
> >> >> >> >> >>  kernel/sched/fair.c | 5 +++++
> >> >> >> >> >>  1 file changed, 5 insertions(+)
> >> >> >> >> >>
> >> >> >> >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> >> >> >> >> index ae7ceba..0a26132 100644
> >> >> >> >> >> --- a/kernel/sched/fair.c
> >> >> >> >> >> +++ b/kernel/sched/fair.c
> >> >> >> >> >> @@ -7004,6 +7004,11 @@ struct task_struct *
> >> >> >> >> >>         struct task_struct *p;
> >> >> >> >> >>         int new_tasks;
> >> >> >> >> >>
> >> >> >> >> >> +       if (prev &&
> >> >> >> >> >> +           fair_policy(prev->policy) &&
> >> >> >> >> >
> >> >> >> >> >Why do you need a prev and fair task  ? You seem to target the special
> >> >> >> >> >case of pick_next_task  but in this case why not only testing rf!=null
> >> >> >> >> > to make sure to not return immediately after jumping to the idle
> >> >> >> >>
> >> >> >> >> >label?
> >> >> >> >> We just want to do load_balance only when CPU switches from SCHED_NORMAL
> >> >> >> >> to SCHED_IDLE.
> >> >> >> >> If not check prev, when the running tasks are all SCHED_IDLE, we would
> >> >> >> >> do newidle_balance everytime in pick_next_task_fair, it makes no sense
> >> >> >> >> and kind of wasting.
> >> >> >> >
> >> >> >> >I agree that calling newidle_balance every time pick_next_task_fair is
> >> >> >> >called when there are only sched_idle tasks is useless.
> >> >> >> >But you also have to take into account cases where there was another
> >> >> >> >class of task running on the cpu like RT one. In your example above,
> >> >> >> >if you replace the normal task on CPU2 by a RT task, you still want to
> >> >> >>
> >> >> >> >pick the normal task on CPU1 once RT task goes to sleep.
> >> >> >> Sure, this case should be taken into account,  we should also try to
> >> >> >> pick normal task in this case.
> >> >> >>
> >> >> >> >
> >> >> >> >Another point that you will have to consider the impact on
> >> >> >> >rq->idle_stamp because newidle_balance is assumed to be called before
> >> >> >>
> >> >> >> >going idle which is not the case anymore with your use case
> >> >> >> Yes. rq->idle_stamp should not be changed in this case.
> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >> Actually we want to pull a SCHED_NORMAL task (if possible) to run when a cpu is
> >> >> >> about to run SCHED_IDLE task. But currently newidle_balance is not
> >> >> >> designed for SCHED_IDLE  so SCHED_IDLE can also be pulled which
> >> >> >> is useless in our situation.
> >> >> >
> >> >> >newidle_balance will pull a sched_idle task only if there is an
> >> >> >imbalance which is the right thing to do IMO to ensure fairness
> >> >> >between sched_idle tasks.  Being a sched_idle task doesn't mean that
> >> >> >we should break the fairness
> >> >> >
> >> >> >>
> >> >> >> So we plan to add a new function sched_idle_balance which only try to
> >> >> >> pull SCHED_NORMAL tasks from the busiest cpu. And we will call
> >> >> >> sched_idle_balance when the previous task is normal or RT and
> >> >> >> hoping we can pull a SCHED_NORMAL task to run.
> >> >> >>
> >> >> >> Do you think it is ok to add a new sched_idle_balance?
> >> >> >
> >> >> >I don't see any reason why the scheduler should not pull a sched_idle
> >> >> >task if there is an imbalance. That will happen anyway during the next
> >> >>
> >> >> >periodic load balance
> >> >> OK. We should not pull the SCHED_IDLE tasks only in load_balance.
> >> >>
> >> >>
> >> >> Do you think it make sense to do an extra load_balance when cpu is
> >> >> about to run SCHED_IDLE task (switched from normal/RT)?
> >> >
> >> >I'm not sure to get your point here.
> >> >Do you mean if a sched_idle task is picked to become the running task
> >> >whereas there are runnable normal tasks ? This can happen if normal
> >> >tasks are long running tasks. We should not in this case. The only
> >> >case is when the running task, which is not a sched_idle task but a
> >> >normal/rt/deadline one, goes to sleep and there are only sched_idle
> >> >tasks enqueued. In this case and only in this case, we should trigger
> >> >a load_balance to get a chance to pull a waiting normal task from
> >> >another CPU.
> >> >
> >> >This means checking this state in pick_next_task_fair() and in balance_fair()
> >>
> >> We made another change would you please give some comments?
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 04a3ce2..2357301 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -7029,6 +7029,10 @@ struct task_struct *
> >>         struct task_struct *p;
> >>         int new_tasks;
> >>
> >> +       if (sched_idle_rq(rq) && prev && prev->state &&
> >> +           prev->policy != SCHED_IDLE)
> >> +               goto idle;
> >> +
> >>  again:
> >>         if (!sched_fair_runnable(rq))
> >>                 goto idle;
> >> @@ -10571,7 +10575,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >>          * We must set idle_stamp _before_ calling idle_balance(), such that we
> >>          * measure the duration of idle_balance() as idle time.
> >>          */
> >> -       this_rq->idle_stamp = rq_clock(this_rq);
> >> +       if (!rq->nr_running)
> >> +               this_rq->idle_stamp = rq_clock(this_rq);
> >
> >I know that I asked you to take care of not setting idle_stamp during
> >the last review. But I forgot that it was cleared anyway at the end of
> >newidle_balance() if there is some tasks running on the cpu so this is
>
> >not needed and make the code less readable
> Yes, the idle_stamp was cleared.
>
> >
> >>
> >>         /*
> >>          * Do not pull tasks towards !active CPUs...
> >>
> >
> >I don't see the change for balance_fair()
> >When a rt task goes back to sleep and there is only sched_idle tasks
>
> >as an example
>
>
> Yes, we should consider this situation too.
> How about this one ?
>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04a3ce2..982b842 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6849,6 +6849,9 @@ static void task_dead_fair(struct task_struct *p)
>  static int
>  balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
> +       if (sched_idle_rq(rq))
> +               return newidle_balance(rq, rf) != 0;
> +
>         if (rq->nr_running)
>                 return 1;
>
Maybe we'd better merge the branches? like,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ac950ac950bc..259deda79c06 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6852,7 +6852,7 @@ static void task_dead_fair(struct task_struct *p)
 static int
 balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
-       if (rq->nr_running)
+       if (rq->nr_running && !sched_idle_rq(rq))
                return 1;

        return newidle_balance(rq, rf) != 0;

Thx.
Regards,
Jiang